Design, synthesis, and performances of double-shelled LiNi_(0.5)Co_(0.2)Mn_(0.3)O_2 as cathode for long-life and safe Li-ion battery

摘要

LiNi_(0.5)Co_(0.2)Mn_(0.3)O_2 is redesigned into a new core-shelled Li[(Ni_(0.8)Co_(0.1)Mn_(0.1))_(2/7)]_(core)[(Ni_(1/3)Co_(1/3)Mn_(1/3))_(3/14)]_(inner-shell)[(Ni_(0.4)Co_(0.2)Mn_(0.4)_(1/2)]_(outer-shell)O_2, in which LiNi_(0.8)Co_(0.1)Mn_(0.1)O_2 may deliver high capacity and LiNi_(0.4)Co_(0.2)Mn_(0.4)O_2 provides structural and thermal stability. To achieve such designed structure, double-shelled hydroxide precursors are firstly prepared via a co-precipitation route. Scanning electron microscope (SEM) shows that all precursors are of 6-10 μm spherical secondary particles developed from nanosheet-shaped primary particles. Energy disperse X-ray spectrum (EDS) on the surface of precursors, in combination with increase of particles size from core to shell during co-precipitation process, confirms the formation of core-shell structure as designed. The spherical morphology is preserved after lithiation at different temperatures from 800 ℃ to 900 ℃ while the morphology of primary particles changes from nano-sized plate to micron-sized rectangular-like shapes. EDS surface composition analysis of lithiated compounds also strongly suggests the formation of core-shell structure; nevertheless, diffusion of transition metal ions between the core and shell occurs and becomes severe with increase of sintering temperature. Consequently, the double-shelled materials especially prepared at 850 ℃ display the remarkably improved cycleability, rate capability, and thermal stability in contrast to normal one. The enhancement of those properties may be ascribed to structurally stable double shell components, especially outer shell.